The THUMS oil islands offshore of Long Beach, one of which is picture here, were found to be in compliance by the Air Quality Management District as it went looking for the source of the rotten-egg odor that periodically plagues the coast. Photo by Chris Carlson/Associated Press.

The mysterious odor has intermittently plagued the coast for years, with AQMD doggedly investigating its origin. The agency has yet to definitively identify the source, although air samples indicate it may be related to offshore petroleum rigs, tankers or other oil operations.

Experts says it’s unlikely to pose a long-term health risk, but it continues to annoy residents and visitors with no end in sight.

“It’s a documented problem,” said Joseph Lyou, president of the Coalition for Clean Air and an AQMD board member. “This odor has been very frustrating for AQMD.”

What is the smell?

It’s most likely hydrogen sulfide, which has a pungent sulfur odor. Hydrogen sulfide is found in oil fields and other petroleum operations, and can arise from rotting organic matter including seaweed, algae blooms and dead fish.

Mercaptan, which has a similar odor, is added to natural gas and other odorless toxins to help quickly identify potentially dangerous leaks. But AQMD officials suspect the smell is usually hydrogen sulfide because the chemical has shown up in air tests.

What’s known about the source of the most recent stink?

AQMD is investigating whether part of the problem stems from a leak of sulfur hexafluoride Sunday at an Edison substation in Huntington Harbour.

The chemical is often used in high-voltage electrical equipment. A report filed with the state shows that 5 pounds of the compound leaked from the substation.

But because the recent flood of complaints started before the leak, AQMD is looking for other sources as well. Since the odors typically occur near the coast while winds are blowing from the ocean toward land, the primary source or sources are thought to be offshore.

Could it be methane, a.k.a. natural gas?

The Huntington Beach Fire Department wrote on its Facebook page that the source was a naturally occurring “methane plume” from the ocean. But methane is not the chief culprit suspected by AQMD.

Methane in its 100-percent natural state has no odor, according to Ed Avol, a USC professor of environmental health.

“However, methane deposits in the real world are rarely found in a 100-percent pure state, and are often co-located with other gases such as hydrogen sulfide, which has a strong ‘rotten eggs’ smell,” said Avol. “So when extracted from wells and such, one gets a combination of methane and hydrogen sulfide.”

Why do experts think it may be related to petroleum operations?

AQMD regularly tests the air during odoriferous episodes and “some compounds have had a petroleum signature,” said AQMD spokesman Sam Atwood.

Hydrogen sulfide is found in crude petroleum.

Has AQMD targeted petroleum operations in its investigations?

Yes. AQMD has tested the oil rigs offshore of Long Beach and found that they complied with all regulations, Atwood said.

The agency also uses infrared cameras to determine if significant levels of emissions coming from refineries, oil tankers and petroleum-loading terminals.

If there’s a leak, how big might it be?

Not necessarily very big.

“These odors are detected at very low levels,” Avol said. “You can smell it almost before you can measure it.”

There is no indication that the compound is at those concentration levels.

Nine months of AQMD testing in 2017 found that levels were below those at which short-term headaches and nausea can occur in some people. Hydrocarbons also sometimes occurred in levels higher than “typical” but below thresholds the state has determined are likely to cause health problems, according to the AQMD website.

“We don’t think this poses any long-term health issues,” Atwood said. “As with any odor issue, there can be short-term effects. And there’s a quality of life issue.”

What should people do if they detect the rotten-egg smell?

“They should definitely call us and let us know at 800-CUT-SMOG,” Atwood said. “It’s important they call us immediately when we smell it, not days later.”

If side effects are felt — or if someone simply wants to get away from the annoying smell — they can go inside, close the windows and turn on the air conditioner, he said.

This story was updated to clarify some details about hydrogen sulfide.

Martin Wisckol covers coastal environment and development. Previously, he spent two decades as politics reporter and columnist for the Orange County Register. He’s also held reporting positions in Miami, Jacksonville, Detroit and his hometown of San Diego, with an emphasis on land use and urban planning. He is a lifelong surfer and has spent most of his life on the coast. His work has been honored by the Society of Professional Journalists, the National Headliner Awards, the California Newspaper Publishers Association, the Florida Press Club and the American Planning Association Florida Chapter.

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